The envelope glycoprotein (Env) of HIV mediates virus entry by undergoing complex conformational changes while it binds to target cells. Our research goals are to elucidate the molecular details of the structural changes that occur in Env as it fuses viral and cellular membranes. Our projected focus on helical segments in the transmembrane protein (TM) of Env that have been recently shown to form a stable, alpha helical bundle. Our studies of these domains include: Site-directed mutagenesis of these regions to determine effects on fusion activity and to correlate alpha bundle stability with fusion activity; Generation and characterization of monoclonal antibodies specific to these helical regions in TM to determine when these regions are exposed during virus entry and to determine if antibodies raised to transient Env conformations can be neutralizing. This latter point may help to determine whether novel neutralizing antibodies can be raised by immunizing with antigens in altered conformations and provide insights into the structural basis of antibody recognition. To date, our mutagenesis studies indicate that single amino acid substitutions, not predicated to disrupt overall helical structure, can profoundly inhibit Env activity. In addition, a monoclonal antibody raised against a helical component of the TM indicates that the helical structure is not accessible in native Env.